1. Field of the Invention
This invention relates to digital printing of pages containing variable information, with unrestricted variability from page to page. More particularly this invention relates to a method and an apparatus for merging a plurality of compressed data, the merging being carried out without decompressing the data.
2. Description of the Problem
Recent digital printing devices have made it possible to print page sequences where each printed page is completely different from the previous one. This is of particular importance in the production of personalized printed matter, e.g., for direct mailing purposes. Such a system for printing personalized multi-page documents should have the following features:
the possibility to store parts of those documents for reuse in other documents as page elements; PA1 an as good as unlimited number of pages in those documents (up to several hundreds of pages); PA1 no inherent restrictions to the amount of personalized data per page; and PA1 a high degree of freedom in the design of the personalization (a large number of regions, overlapping regions, etc.) PA1 a single master is RIPed and stored as an uncompressed bitmap in memory; PA1 with special postscript instructions, parts of that bitmap are copied to a cache; PA1 the postscript code for the variable data is RIPed and parts of the cached master bitmap are overwritten; and PA1 after printing the parts of the master bitmap that were changed are restored.
Such a printing system can be conceptually described as a page element based printing system: a document is created from pages, and each page is created from a number of elements stored in the system in a compressed raster form. In a typical workflow in which the present invention may be used, a prepress system produces the overall description of pages and of the graphical data elements which are the page elements.
A page is typically described using a page description language ("PDL") such as the PostScript.RTM. PDL and "PDF.RTM." by Adobe Systems, Inc., Mountain View, Calif., HP PCL by Hewlett-Packard, Inc., Palo Alto, Calif., or by a format such as GRO.TM. ("GRaphics Objects"), the native internal format used by Barco Graphics NV, Gent, Belgium, the assignee of the present invention. A PostScript file for instance contains a sequential list of commands, which, when interpreted, generate graphical objects such as line work (including text), CT images, paths (a set of vector or spline segments that describe a shape or set of line segments), bounding boxes ("invisible" rectangles that define the boundaries of an object), etc. Paths may have attributes such as stroke and fill, and may define, among other things, bounds of objects, objects themselves, and clipping masks which, when associated with an object, define which part of the object is visible. Such concepts would be well known to those of ordinary skill in the art. PostScript interpreting typically generates a display list of objects, and these objects normally are rasterized prior to printing or display. In the same way a display list of objects can be generated by interpreting page description files in other page description languages. The combined interpreting of the PDL and rasterizing is referred to as raster image processing ("RIPing") and the software and/or hardware system for carrying this out is called a raster image processor ("RIP").
The system driving the print engine must generate a data stream at the speed of the print engine. This means that the data rate of the system is very high, and fully dictated by the imaging speed and the resolution of the printing devices. The Xeikon DCP 32D (Xeikon NV, Mortsel, Belgium), for instance, prints full duplex cyan, magenta, yellow and black ("CMYK") sheets of A4 size at 600 dots per inch and with 4 bits per spot per separation, at a speed of 35 duplex (i.e., double sided) sheets per minute. The output bandwidth necessary to drive this device is over 80 Mbytes per second. If we assume a 50% overlap of the variable objects with the background master object, the raster image bandwidth must be at least 120 Mbytes per second. The I/O bus bandwidth needed is the sum of the input and output bandwidth, plus the bandwidth needed by the merge process and other processes. So the I/O bus bandwidth of the computer system must be more than 200 Mbytes per second, which is well beyond the reach of the standard computer systems that are available today. In the near future even faster printing devices will become available. In the last few years, the increase in data rates of printing devices has outpaced the increase in available affordable computing power.